The present invention relates to a tire having a radial carcass reinforcement, and more particularly to the constitution of the beads of such tires.
European Patent EP 0823341 describes a tire comprising a crown reinforcement surmounted by a tread, two beads and at least one radial carcass ply extending from one bead to the other. Each bead of said tire has the following characteristics:
1) it is devoid of bead wire, in the sense that the person skilled in the art understands xe2x80x9cbead wirexe2x80x9d to mean a reinforcing element which generally fulfills the two functions of anchoring the carcass reinforcement at a given inflation pressure and of clamping the beads on the rim;
2) it comprises an annular element, the ultimate tensile strength of which in the longitudinal direction is significantly less than that which would be necessary for a conventional bead wire used in a tire of the same dimension, but which is however sufficient to permit the operations of building, turning-up, shaping and vulcanization which are effected during the manufacture of the tire;
3) the mechanical strength necessary for anchoring the carcass reinforcement is generally provided by two layers referred to as bead reinforcement layers, arranged in contact with the annular element, or close to said element, said reinforcement layers each comprising reinforcing elements which are parallel to each other within each layer and are crossed from one layer to the next, forming an angle greater than 0xc2x0 and at most equal to 10xc2x0 with the circumferential direction.
The beads of the tire which are thus formed make it possible to use a mix of low hysteresis loss and lesser thickness as axially outer and radially inner rubber layer intended to provide the connection to the mounting rim. The result is a reduction in weight of the tire, and a significant reduction in the rolling resistance while keeping the speed and endurance properties at a high level.
Nevertheless, the tires in question do not have perfect ability to be mounted on a rim regularly over time: many tires mount easily and correctly, whereas some tires which are allegedly identical have mounting defects which have to be corrected.
In order to overcome these drawbacks, the invention proposes a better distribution of the tensile strengths of the elements which constitute the bead. Thus the tire, according to the invention, comprises a crown reinforcement surmounted by a tread, said tread being joined by means of two sidewalls to two beads, between which extends at least one carcass reinforcement ply, anchored within each bead by winding around an annular element, the tensile strength of which is less than the tensile strength of a bead wire of a conventional tire of the same dimension, and an assembly of at least two bead reinforcement layers which are arranged in contact with the annular element and are composed of reinforcing elements which are parallel to each other within each layer and are crossed from one layer to the next, forming an angle xcex1 satisfying the relationship 0xc2x0 less than xcex1xe2x89xa615xc2x0 with the circumferential direction. It is characterized in that the annular element has a tensile strength of between 3 and 5 times the tension imparted to the carcass reinforcement by the recommended inflation pressure, and the elongation at break of which is between 2 and 6%, all the bead reinforcement layers having a tensile strength of between 0.5 and 1 times the tensile strength of the annular element, and the total of the tensile strengths of the annular element and the assembly of the reinforcement layers being between 6 and 8 times the tension imparted to the carcass reinforcement.
xe2x80x9cTension imparted to the carcass reinforcement by the recommended pressurexe2x80x9d is to be understood to mean the tension calculated per unit of length exerted in the plane of the bead wire.
The annular element is preferably metallic and more particularly made of steel, whereas the bead reinforcement layers are advantageously formed of cables of aromatic polyamide in order to permit the lesser weight, two reinforcement layers possibly being the two strands of a ply which is turned up about the annular element, or possibly being two axially adjacent plies.
xe2x80x9cTensile strengthxe2x80x9d of an assembly of layers is to be understood to mean the ultimate strength obtained by traction exerted on a sample of all the layers which is taken from the new, vulcanized tire, which traction is exerted along an axis corresponding to the circumferential direction of said tire.
As is known, the layer of rubber mix arranged axially and radially to the outside of the bead and intended to provide the connection with the rim preferably has a thickness of at most 2 mm, and the rubber mix of which it is composed a modulus of elastic loss Gxe2x80x3 less than 1, Gxe2x80x3 being expressed in MPa (megapascals), and Gxe2x80x3 being measured at 10% shear, at 50xc2x0 C. and at a frequency of 10 Hz.